The UK is looking "very carefully" at the protection offered by the Pfizer/BioNTech Covid-19 vaccine because of concerns that one dose is less effective than initially thought.
The country's chief science adviser Sir Patrick Vallance was on Wednesday asked about an Israeli study that found people who had received one dose were only 33 per cent less likely to test positive for Covid than people who had not.
The researchers claim this figure casts doubt on the UK's mass vaccination strategy, which uses the Pfizer/BioNTech product.
Israel leads the world in its vaccination efforts and its approach could provide a valuable framework for other countries. It has administered almost 2.8 million doses, the highest global inoculation rate per capita.
"Things are seldom as good as in clinical trials," Mr Vallance told Sky News. "We need to look at this very carefully, we just need to keep measuring the numbers."
According to UK health chiefs, one dose of the vaccine could offer up to 89 per cent protection. That prediction was used to justify delivering the vaccine to as many people as possible, with a wait of up to 12 weeks for a second dose.
Mr Vallance said the discrepancy might be because the drug generates a strong response by a typical human immune system against the virus, but only 10 days after injection.
Mr Vallance said a single Pfizer/BioNTech dose could conceivably provide 89 per cent efficacy, but admitted it “probably won’t be as high as that in practice".
Israeli scientist and epidemiologist Professor Ran Balicer cast some doubt on the UK's trial data.
"We could not see a 89 per cent reduction in the data we reported. Further data and analyses will be released in peer reviewer scientific format."
Separately, Mr Vallance said introducing a more stringent lockdown sooner could have prevented further deaths in the UK.
"We will have got some things right and some things wrong and we've learnt a lot as we've gone through this," he said.
"The lesson is go earlier than you think you want to, go a bit harder than you think you want to, and go a bit broader than you think you want to in terms of applying the restrictions.
"It's worth remembering the definition of insanity by Einstein, which is doing the same thing over and over again and expecting for a different outcome.
"The lesson is every time you release it too quickly you get an upswing and you can see that right across the world."